But on the eve of the March 1 deadline it seems unlikely to happen to schedule.

Logistical hurdles

"We are still waiting for some key data from the SPLM-IO and for them to assemble their forces," said a source close to the implementation process, talking on the condition of anonymity.

"The SPLM-IO has said it won't come to Juba until support requirements are put in place, such as food, water supply and shelter," said the source. "They also have to decide who they want to come, where they are coming from, and make sure they are assembled in the right place."

The 1,370 SPLM-IO personnel are coming from a variety of locations. "Some are coming from Pagak [the SPLM-IO headquarters, close to the border with Ethiopia] and some from other areas of Upper Nile," said James Gatdet Dak, a spokesman for the SPLM-IO in Nairobi.

Moving so many people around a country that has extremely basic transport infrastructure is a considerable challenge.

"It will have to be an elaborate operation," said a source close to the Intergovernmental Authority on Development (IGAD), the regional trade body that was responsible for the mediation of the August peace agreement, who asked to remain anonymous.

"A good chunk of the men are in Pagak," said the IGAD source. "They will move to Gambella [in Ethiopia], and from there to Addis Ababa, and then on to Juba."

Many of these men will have to travel by road from Pagak to Addis Ababa, a distance of more than 800km.

Peace process barriers

A few days' delay will not derail a peace process that has already suffered numerous postponements. The government of national unity was initially due to be formed in January.

"It might take a while to organise the return of SPLM-IO troops to Juba, but once these fellows make up their mind, it can happen very quickly," said the source close to IGAD.

But more serious than the effect of logistical issues is the potential for further political obstacles to impede the implementation of the deal.

"The matter of transport is relatively straightforward," said the source close to the implementation process. "The logistics are less complicated than the politics."

The eventual agreement on a transitional government of national unity (TGONU) in South Sudan in August was the result of more than 18 months of negotiations, and there continue to be barriers to implementation at every stage.

In recent weeks, despite ongoing delays, there have been signs of progress. The government has committed to working towards the evacuation of the bulk of its troops beyond a 25km radius of Juba, one of the requirements of the August agreement, and a key demand of the SPLM-IO.

"We commend it," said Gatdet Dak. "We are still waiting for it to be verified. It seems that some of the troops have not gone as far as 25km, and some troops are coming back to Juba at night."

"It's a process," said minister of information Michael Makuei Lueth. "We will leave a limited force to secure Juba until the time when the SPLM-IO forces arrive."

Security in Juba is paramount not only to a political agreement, but also to ensuring that there is no repeat of the fighting in the capital that sparked the conflict that has consumed South Sudan for more than two years.

Once the SPLM-IO security forces have reached Juba, Riek Machar is due to follow. "It will not take many days," says Gatdet Dak. "He will come by plane from Pagak."

Machar, who was ousted from his position as vice president in July 2013 by the president, Salva Kiir, has not been to Juba since December 2013, when clashes within the presidential guard escalated into a war that took on a tribal dimension.

Tens of thousands of people have since been killed, and more than two million displaced from a country that only came into existence in July 2011, when it gained independence from Sudan.

In accordance with the terms of the August deal, on February 11 Kiir issued a republican decree appointing Machar to the position of first vice president.

The SPLM-IO will also nominate 10 ministers to the TGONU, although these are yet to be confirmed.

"Dr Riek has a list of ministers, but he wants to submit them in person once he reaches Juba," said Gatdet Dak. "He wants to meet Salva Kiir, and to consult with the leadership of the SPLM-IO, some of whom are in Juba."

Assuming the TGONU can be formed, one of the first items on the agenda will be the resolution of disagreements over the administrative division of the country.

In December, Kiir increased the number of states from 10 to 28, contravening the August peace agreement. The SPLM-IO has been vociferously opposed to the move, initially refusing to send its representatives back to Juba before the issue had been resolved.

In late January, IGAD issued a communique urging the suspension of the implementation of the 28 states pending a review by a boundary commission including representatives from all parties to the peace agreement.

This has opened the way for the return of SPLM-IO forces, and Machar, to Juba. "We're not happy, but we have accepted it," says Gatdet Dak.